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Chapter 5 DICTIONARY CODING
Dictionary Coding
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Dictionary coding is different from Huffman coding and arithmetic coding.
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Both Huffman and arithmetic coding techniques are based on a statistical model (e.g., occurrence probabilities).
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In dictionary-based data compression techniques,
a symbol or a string
of symbols generated from a source alphabet is represented by an
index
to a dictionary constructed from the source alphabet.
Dictionary Coding
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A dictionary is a list of symbols and strings of symbols.
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There are many examples of this in our daily lives.
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the string “September” vs. “9,”
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a social security number vs. a person in the U.S.
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Dictionary coding is widely used in text coding.
Dictionary Coding
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Consider English text coding.
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The source alphabet includes 26 English letters in both lower and upper cases, numbers, various punctuation marks and the space bar.
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Huffman or arithmetic coding treats each symbol based on its occurrence probability. That is, the source is modeled as a memoryless source.
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It is well known, however, that this is not true in many applications.
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In text coding,
structure
or
context
plays a significant role.
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Very likely that the letter
u
appears after the letter
q
.
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Likewise, it is likely that the word “concerned” will appear after “As far as the weather is.”

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